Dissertation (Ph.D.)

Virginia Polytechnic Institute and State University

Abstract (academic):

Current treatment of wastewater can effectively remove the contaminants; however, the effluent is still not widely reused because of some undesired substances like pathogens and trace organic chemicals. To promote water reuse, membrane-based technologies have emerged as a robust and more efficient alternative to current treatment practice. Among these membrane processes, forward osmosis (FO) utilizes an osmotic pressure gradient across a semi-permeable membrane to reclaim high-quality water. Still, several key challenges remain to be addressed towards broader FO application, including energy-intensive draw regeneration to yield product water and salinity buildup in the feed solution. To bypass energy-intensive draw regeneration, commercial solid fertilizers was utilized as a regeneration-free draw solute (DS), harvesting fresh water towards direct agricultural irrigation. However, using nutrient-rich fertilizers as DS resulted in an elevated reverse solute flux (RSF). This RSF, known as the cross-membrane diffusion of DS to the feed solution, led to deteriorated solute buildup on the feed side, reduced osmotic driving force, increased fouling propensity, and higher operation cost. To effectively mitigate solute buildup while achieving energy-efficient water reclamation, a parallel electrodialysis (ED) device was integrated to FO for DS recovery in the feed solution. The salinity in the feed solution was consistently controlled below 1 mS cm-1 via the hybrid FO-ED system. Considering solute buildup is merely a consequence of RSF, direct control of RSF was further investigated via operational strategy (i.e., an electrolysis-assisted FO) and membrane modification (i.e., surface coating of zwitterion-functionalized carbon nanotubes). Significantly reduced RSF (> 50% reduction) was obtained in both approaches with minor energy/material investment. With two major bottlenecks being properly addressed for energy-efficient water reclamation, FO was further integrated with a microbial electrolysis cell (MEC) to achieve integrated nutrient-energy-water recovery from high-strength wastewater (i.e., the digestor centrate). The abovementioned research projects are among the earliest efforts to address multiple key challenges of FO during practical application, serving as a cornerstone to facilitate the transformation of current water/wastewater treatment plant to resource recovery hub in order to ensure global food-energy-water security.

Abstract (general audience):

Exploring alternative water supply, for instance via reusing wastewater, will be essential to deal with the global water crisis. Current wastewater treatment can effectively remove the contaminants; however, the treated wastewater is still not widely reused due to the possible presence of residual contaminants. In recent years, membrane-based technologies have emerged as a promising treatment process to produce clean water. Among all available membrane technologies, forward osmosis (FO) takes advantage of the osmotic pressure difference across a special membrane to extract fresh water from a low-salinity FEED solution (for example, wastewater) to a high-salinity DRAW solution. The reclaimed fresh water can be reused for other applications. Still, the FO process is facing several critical challenges for broader applications. The first challenge is that additional energy is required to separate clean water from the diluted DRAW solution, leading to notably increased energy consumption for the FO process. To bypass this energy-intensive separation, commercial solid fertilizers was utilized as a separation-free DRAW solution for FO process. Once the clean water is extracted to the DRAW solution (fertilizer), the diluted fertilizer solution together with the fresh water can be directly used for agricultural irrigation. The second challenge is that, when fertilizer is applied as the DRAW solution, nutrient-rich fertilizers can penetrate the FO membrane and escape to the FEED solution (wastewater). This phenomenon is known as the reverse solute flux (RSF). RSF can result in many adverse effects, such as wastewater contamination and increased operational cost. To prevent this, we used an additional device named electrodialysis to effectively recapture the “escaped” fertilizers in the FEED solution. Besides this indirect approach to recover escaped fertilizers, we also investigated direct approaches to control RSF, including operational strategy and membrane modification. With two major challenges being properly addressed for energy-efficient water reclamation, FO was further combined with a microbial electrolysis cell (MEC) to achieve multiple resource recovery from wastewater, including water, nutrient, and energy parts. The abovementioned research projects are among the earliest efforts to address multiple key challenges of FO during water and resource recovery from wastewater to ensure global food-energy-water security.


Thesis (M.Sc. of Chemistry)

The Synthesis and Application of Energy Storage Materials Derived from Small Molecules

National University of Singapore / Singapore-Peking-Oxford Research Enterprise (SPORE)

Abstract:

Nowadays, people are paying increasing attention to energy and environmental issues. Developing materials, which can be used in energy-related fields, are commonly recognized as one of the most important human endeavors for sustaining growth.

In order to shine some light on potential approaches to energy-related issues, several kinds of organic and inorganic materials, primarily derived from small molecules, were successfully synthesized in this study and systematically investigated in energy-related application: Li-ion battery, Na-ion battery and photocatalysis.

The results obtained indicates that the small organic molecule, 1,3,5-tri(9H-carbazol-9-yl)benzene (TCB), is highly promising as a candidate material to be deployed as anode in the lithium battery system. With high initial specific capacity (800-900 mAh/g), relatively stable rate and cycling performance and nearly 100 % coulomb efficiency, the novel small organic molecule, TCB, is a suitable anode material in Li-ion battery system.

The organic polymer, micro-wire polycarbazole (PTCB), which is primarily derived from TCB, also presents some electrochemical features with relatively low specific capacity and cycling stability. However, PTCB’s performance as a photocatalyst is outstanding. By using acetonitrile as the solvent and providing adequate oxygen, the conversion rate and selectivity achieve 100 % and 98 %, respectively, after 2 hours’ visible-light exposing. Obviously, PTCB shows a bright future as an effective photocatalyst in photochemical synthesis. Thus, both the TCB and PTCB, as novel organic materials, would contribute much to energy-related applications.

The novel inorganic Sb-carbon composite, which is derived from small organic molecule (triphenylstibane), presents excellent electrochemical performance as the anode material in the sodium-ion battery system. Containing carbon (52 %) and antimony (35 %), this material exhibits large specific capacities (800 mAh/g @ 1C, 630 mAh/g @ 2C and 580 mAh/g @ 4C). Besides, it displays rather stable performance and high (>99%) coulombic efficiency under different rates. Thus, this material is highly promising for application in sodium-ion battery.


Thesis (M.Sc. of Environmental Engineering)

High-efficient nitrogen removal by enriched psychrotolerant autotrophic-nitrification and aerobic-denitrification consortiums at cold temperature

Peking University / Singapore-Peking-Oxford Research Enterprise (SPORE)

Abstract: 

Due to rapid development of economy and modernization, China now suffers from lots of environmental issues, of which eutropication is one of the most serious problems regarding water pollution. Excess nitrogen in the natural receiving water bodies, which is usually caused by insufficient treatment of discharging water in wastewater treatment plants (WWTPs), may contribute much to eutrophication in surrounding environment. In order to strengthen the nitrogen removal efficiencies in most biological treatment facilities, the factors which led to deterioration of simultaneous nitrification and denitrification (SND) were deeply investigated for years, among which the seasonal temperature drops of temperate zones in winter were commonly recognized as a key inhibitor. In order to strengthen the nitrogen removal efficiency and stability in biological wastewater treatment system, a psychrotolerant autotrophic-nitrifying consortium together with a psychrotolerant aerobic-denitrifying consortium has been successfully enriched and systematically characterized in this study. Futher coupling of both consortiums not only greatly enhanced nitrogen removal in SBRs but also significantly reduced energy consumption and carbon source input, which would be of primary importance for potential practical applications.

First, a psychrotolerant autotrophic-nitrifying consortium was successfully enriched through three 5, 25, 50 L SBRs under the aerobic condition, successively. This autotrophic-nitrifying consortium capable of excellent nitrification performances could reach up to a maximum specific nitrifying rate of 8.85 mg N/ (g SS∙h). Further PCR-DGGE and other microbial analysis determined that the Nitrosomonas sp. was predominant in this consortium.

Afterwards, the bioaugmentation performance of the autotrophic-nitrifying consortium towards normal SBR systems was investigated under cold temperature. Only one-time dosing with a minimum dosage of 0.04 g/L could guarantee an excellent ammonia removal rate (>99 %). Futher long running test proved that the quick integration of this nitrifying consortium can result in a rather stable bioaugmented system with a perfect shock-resistance capability. Even after drying and cold storage process, the nitrifying consortium still presented a great nitrification performance, suggesting a great potential for future engineering applications.

Elimination of nitrogen in wastewater needs both denitrifers and nitrifers. Therefore, coupling of nitrifying consortium with psychrotolerant denitrifying consortium should be a good option to complete total nitrogen removal at low temperature. Therefore, another psychrotolerant aerobic-denitrifying consortium was successfully enriched through 250 mL flask and then 5 L SBR under the aerobic condition. The aerobic-denitrifying consortium could achieve a maximum specific denitrifying rate of 32.93 mg N/ (g SS∙h) under dissolved oxygen of 0.8-1.2 mg/L at 10 °C. Further PCR-DGGE and other microbial analysis determined that the Pseudomonas sp. and Rhodoferax ferrireducens were predominant in this consortium.

Finally, coupling both kinds of consortiums was proved very successful for a perfect TN at COD/N of 4 and dissolved oxygen of 1.5-4.5 mg/L, which was hardly reached by any single consortium reported previously. Nitrogen removal was achieved through simultaneous nitrification and denitrification (SND). The encouraging results from coupling aerobic consortiums implied a huge potential in practical treatment of low-strength domestic wastewater (200-300 mg/L COD) during wintertime.

Keywords: Autotrophic nitrification; Aerobic denitrification; Coupling; Low COD/N; Low temperature TN removal


Thesis (B.Eng. of Environmental Engineering)

Characterization and performance optimization of 3 strains of phosphorus accumulating organisms (PAOs)

Beijing Institute of Technology / Peking University

Abstract: 

A series of experiments has been conducted with three PAOs, previously screened out in our lab, to investigate the influence of various growth factors towards optimized performance. The results will shed some lights in practical implements of wastewater treatment.

The results of lab-scale experiments are as follows: in the high phosphorus concentration experiment the maximum phosphorus removal rate of these three PAOs are 13.8%, 16.5% and 19.2%; while cultivating in the low phosphorus concentration, the maximum phosphorus removal rate are 87.9%, 86.3% and 81.4%. Moreover, through the orthogonal experiment, the optimal phosphorus removal conditions are as the follows: A: pH=8, T=30 °C, r=100 r/min, C/N=12; B: pH=8, T=20 °C, r=100 r/min, C/N=12; C: pH=8, T=30 °C, r=100 r/min, C/N=12.

Through long-term pure and mixed culture incubation, we study the  possibility of using these three PAOs in the EBPR systems. The result indicates that the phosphorus removal effect of long-term pure culture incubation is satisfying (nearly 70% in 400-h incubation), whereas the mixed ones exhibit a deteriorated performance due to interspecific competition.

Key words: Eutrophication; Waste water phosphorus removal; Enhanced biological nutrient removal; Phosphorus accumulating organisms